In recent years, a tire forming method using a rigid core has been proposed in order to improve the accuracy of forming a tire (see Patent Documents 1 and 2, for example). The rigid core has a core main body having an outer shape which accords with a shape of a tire cavity surface of a vulcanized tire. By sequentially applying tire constitutional members onto the core main body, a green tire is formed. Then, by putting the green tire into a vulcanizing mold together with the rigid core, the green tire is vulcanized and molded while being sandwiched between the core main body as an inner mold and the vulcanizing mold as an outer mold.
As shown in FIG. 8(A), the core main body (a) is divided into plurality of core segments (c) in the circumferential direction so as to be disassembled and removed from the tire after the vulcanization molding.
For details, the core segments (c) comprise first core segments (c1) having both circumferential end surfaces as first butting surfaces (sc1) and second core segments (c2) having both circumferential end surfaces as second butting surfaces (sc2). The first core segments (c1) are configured to be smaller in the circumferential width than the second core segments (c2). The first core segments (c1) and the second core segments (c2) are alternatively arranged in the circumferential direction. And the core main body (a) is formed into an annular shape by mutually butting the circumferentially adjacent first butting surfaces (sc1) and second butting surfaces (sc2).
The first butting surface (sc1) is formed as an outwardly inclined surface which is inclined so that the circumferential width increases toward the inside in radial direction. In contrast, the second butting surface (sc2) is formed as an inwardly inclined surface which is inclined so that the circumferential width decreases toward the inside in radial direction. Therefore, the core segments (c) are removed from the tire by being moved radially inwardly one-by-one from the first core segments (c1).
The temperature of the core main body (a) increases from an ordinary temperature state (about 15 to 50 degrees C.) during forming a green tire to a high temperature state (100 degrees C. or more) during vulcanization molding. By such temperature rise, the core main body (a) causes a thermal expansion, and pressure is generated between the circumferentially adjacent core segments (c1), (c2). Owing to the pressure, as shown in FIG. 8(B), the first core segments (c1) having the first butting surface (sc1) as the outwardly inclined surface are pushed radially inwardly whereas the second core segments (c2) having the second butting surface (sc2) as the inwardly inclined surface are pushed radially outwardly. In consequence, there is a problem such that steps (d) in the radial direction are formed between the outer peripheral surfaces of the first core segments (c1) and the second core segments (c2), and the tire is deteriorated in the uniformity.
It is conceivable to increase the amount of gaps between the first butting surfaces (sc1) and the second butting surfaces (sc2) in an ordinary temperature state in order to decrease the steps (d) by suppressing the pressure during vulcanization molding. However, the increase in the amount of gaps has a problem such that, during vulcanization, rubber biting of flow-in rubber becomes liable to occur and the quality of the tire is reduced.
Patent Document 1: Japanese published unexamined application 2011-161896
Patent Document 2: Japanese published unexamined application 2011-167979